In vehicles, drivers generally use an interior rearview mirror and two exterior side view mirrors (hereinafter referred to collectively as “rearview mirrors”). The rearview mirrors allow the driver to view scenes behind the vehicle without having to face in a rearward direction and to view areas around the vehicle that would otherwise be blocked by the vehicle structure. As such, these mirrors are an important source of information to the driver. Bright lights appearing in a scene behind the vehicle, such as from another vehicle approaching from the rear, may create glare in a rearview mirror that can temporarily visually impair or dazzle the operator. This problem is only aggravated under low ambient light conditions such as at night, when the eyes of the driver have adjusted to the darkness.
Various solutions have evolved to deal with the problem of glare in rearview mirrors of vehicles. One conventional solution to this problem, used primarily with interior, center-mounted rear view mirrors, is to employ a prismatic mirror with a switch lever on the mirror housing. The switch can be manually moved between a daytime position, providing direct, normal intensity reflection from the mirror surface, and a nighttime position, providing a reduced intensity reflection. When the driver experiences glare, he manually changes the rearview mirror setting to low reflectivity. With the low intensity of light reflected to the driver, the intensity of reflected headlights from trailing vehicles is insufficient to impair the driver's vision. Once the glare is subsided, the driver can manually switch the rearview mirror back to high reflectivity. Difficulties with manually controlled mirrors include the glare experienced before the mirror could be switched, as well as driver distraction caused by finding and operating the switch lever.
Other solutions include automatically dimming rearview mirrors which eliminate the need for the operator to manually switch the mirror. Improvements in glare reduction occurred when prismatic mirrors having two states were replaced with multi-state mirrors which include dimming elements capable of providing many levels of reflectivity reduction. One type of such multi-state automatically dimming rearview mirror is based on changes in the absorption spectra of some materials due to a change in the oxidation state induced by an external current flow. This effect is referred to in the literature as the electrochromic effect and such automatically dimming rearview mirrors are commonly termed electro-chromic mirrors. An electrochromic mirror includes an electrochromic medium connected between two electrodes. The electrochromic medium is responsive to external current generated by applying charge to a pair of electrodes. When a sufficient electrical current is applied across the electrodes of the automatically dimming rearview mirror, the electrochromic medium enters a tinted state by changing its spectral characteristics. However, electrochromic mirrors suffer from many limitations such as slow response rate, high temperature sensitivity, and high power consumption.
Other known automatically dimming mirrors make use of the properties of liquid crystals. Liquid crystals have very fast response time, lower power consumption, and low temperature sensitivity in useable range. Application of an electric field reorients the liquid crystal molecules and changes their optical properties such as birefringence or absorption. In liquid crystal based dimming systems, when the molecules are in the realignment state, the light reflected from the mirrors is attenuated to a degree that is normally proportional to the applied electric field. Upon reducing or removing the applied electric field, the system returns to a normal, more transparent state. Using such mirrors, therefore, it is possible to obtain selectively a high or a low reflecting power, according to whether the electrical voltage applied to the liquid crystal is lower or greater than the threshold. However, liquid crystal based systems typically use static (not switchable) absorptive polarizers for light attenuation, which often reduces the reflectivity to <50% or <40%, even in the highly reflective state. This low reflectance automatically eliminates their use for many applications including rearview mirrors. More recently, guest host systems have been proposed to overcome this limitation. However, these systems still do not offer the high reflectance or wide swing in the reflectivity between the clear and the dark state that is achievable with electrochromic systems.
To circumvent the issue associated with static absorptive polarizers, the use of switchable polarizers has been proposed. These can be reflective such as the device in U.S. Pat. No. 7,362,505 (Hikmet et al.) or absorptive, such as the device in US Pub. No. 2005/0057701 (Weiss). However, switchable reflective polarizers based on cholesterics do not possess the optical or electrical characteristics needed for many dimmable mirror applications. For example, it is well known that switchable cholesteric polarizers have a high degree of haze in both transmissive and reflective states, which makes them unsuitable for optical applications, particularly when a single light source (such as a headlamp of a car) is used. Furthermore, they require high switching voltage and have an undesirably long relaxation time (e.g. several minutes) to the reflective state. These drawbacks make them unsuitable for dimmable mirror applications.
Absorptive active polarizers have been suggested for autodimming applications (see e.g. Weiss, US Pub. No. 2005/0057701). In these systems, a single active polarizer is used to absorb both polarizations of the incident light. To achieve this, the combination of a quarterwave plate and a highly reflective, polarization independent mirror is used to rotate the polarization of unabsorbed light. The disadvantage of this system is that the reflection will not be uniform across the entire visible spectrum since quarterwave plates are wavelength and angular dependent. As such, the system will exhibit non-uniformity, especially in point source illumination conditions as observed with head lamps at night. In addition, these systems are not compatible for use with displays or other images.
Therefore, there is a need for dimming mirrors that can offer fast response time, low power consumption, and a low temperature sensitivity which also possess a high reflective state and a low dark state with potential for intermediate states.
Similarly, there is a need for electronically dimmable transmissive devices, such as windows in a building, vehicle, airplane, etc., where the amount of light passing through the device can be controlled electronically. Thus, for example, a window can be dimmed in the summer to allow less sunlight into the interior of a building, while during winter months, it can be set to maximum transmissivity to allow more sunlight to enter the building. The present invention provides a description of various novel configurations of an electronically dimmable optical device that can be used in reflective and/or transmissive applications.